This application describes an invention made or partially made in the course of work under a U.S. Government contract, viz Air Force Contract No. F33615-82-C-5045.
Maintenance of metal, composites, and other materials covered or painted with a material of poor thermal conductivity such as, for example, paint, grease, ceramics, and the like is quite often a substantial contributor to the cost of ownership. Thus maintenance of painted surfaces is a substantial contributor to the cost of owning aircraft. Stripping of old paint prior to repainting is frequently a necessary maintenance operation.
Toxic, phenolic paint-stripping solvents are labor intensive and hazardous to use. Acids and abrasives damage airframe exteriors, and residual moisture from solvents trapped by skin seams and rivets promotes corrosion.
Paint-stripping damage to a composite aircraft surface is more severe than to a metallic aircraft surface because the boundary between the paint and the airframe material is less distinct for composite structures.
A non-solvent paint-stripping technique proposed for use involves the use of high energy flashlamps. Whether or not this technique will be found to be useful depends on the extent to which the difference in thermal expansion properties of paint and metal is great enough and operative when exposed to a high-energy flashlamp to break the adhesive seal that bonds the paint to the metal.
Another non-solvent paint stripping method described in patent application Ser. No. 342,787 filed Jan. 26, 1982 now abandoned and assigned to the same assignee as this case and which is incorporated herein as if set out at length, involves removing paint and the like from a substrate by ablation without damage to the substrate or its surface by delivering to the material to be removed laser pulses having a wavelength at which the paint is opaque and a fluence sufficient to ablate or decompose the material without damaging or adversely affecting the substrate or its surface.
Paint is removed by sweeping the paint with a pulsed laser beam. The laser beam must have a wavelength at which the material to be removed is opaque and must be carefully controlled to cause ablation of the material to be removed without damaging the substrate or its surface. For this purpose, the fluence of each pulse of the laser beam delivered to the material must be carefully selected to be sufficient to bring the surface of the paint to steady-state ablation, but insufficient to cause plasma formation with accompanying damage to the substrate.
The laser beam used for paint stripping purposes should have a substantially uniform fluence at the work surface and for stripping paint from composite substrates, a variation in fluence of not greater than about ten percent. Further, the beam should be formed to have a reasonable depth of focus to permit substantially uniform treatment of curved surfaces and should have a roll-off of intensity at its edges to reduce the possibility of over exposure in those regions where the direction of the laser beam to a new region overlaps a portion already treated.
The formation of such a laser beam at the work surface that is not subject to asymmetry and aberrations, even if they exist in the output beam of the laser, and the cross section of which is independent of the shape and intensity distribution of the laser beam at the output of the laser is particularly desirable, especially when an inner portion of the output beam of the laser is obscured as is the case when the laser beam is generated with an unstable resonator. It is further desirable, as in the case of paint stripping, for example, that any laser beam shaping apparatus, since it will most likely be carried at the tip of a robotic arm, be as small and light as possible while still providing a useful depth of field.
It is a principal object of this invention to provide a method of shaping a laser beam and to provide apparatus for carrying out this method that accomplishes all of the above-noted objects.